BLACK HOLE BURPS

After black holes devour stars, sometimes the feast comes back up

- YVETTE CENDES

BLACK HOLES ARE INVISIBLE, yet they are among the brightest things in the universe. If a star wanders too close to a black hole, it gets torn apart in a fireworks show called a tidal disruption event. As the star approaches, it gets twisted and pulled, and about half of it ends up flung outward. The other half forms a Frisbee-shaped accretion disk around the black hole itself. This newly formed disk is not stable: material sloshes around and smashes into itself, creating a light show detectable in radio wavelengths.

These are rare occurrences—scientists estimate that the giant black hole at the center of our Milky Way galaxy gobbles a star about every million years or so. But when it happens, it releases a tremendous amount of light and energy visible millions or even billions of light-years away.

Until recently, astronomers had thought that after the initial feast, the swallowed star was never to be seen again. Observations in the past five years, however, suggest otherwise. In a surprising turn unpredicted by theory, it appears that black holes can suffer from indigestion, spewing out material years after the initial stars were shredded. In fact, scientists are now finding that up to half of black holes that devour stars start shining again in radio light years after they had gone quiet—the equivalent of a cosmic burp. We know this material isn’t coming back from beyond the event horizon—that’s impossible. It’s most likely sloshing about in an accretion disk outside that boundary. But explaining how these black hole burps can occur so late is challenging. What’s going on? Solving the mystery of these regurgitations may reveal new secrets about the physics of the most extreme environments in the universe.